Tape storage systems remain the most efficient and cost-effective means for providing data backup because no other storage technology offers the same low cost and high capacity combined advantage. In addition, tape storage systems have been proven to be very reliable.
By combining the advantages of linear multi-channel bi-directional tape formats in common usage, Linear Tape-Open (LTO) technology has been developed to maximize capacity and performance of tape storage systems. LTO tapes use a tape format that has longitudinally pre-written servo tracks. The servo tracks provide a timing-based track-following position error scheme. The servo tracks contain a repeated pattern of recorded flux transitions that occur in grouped bursts of 5, 5, 4 and 4 transitions. The timing between the sets of five-bursts and between sets of four-bursts provides the position information for track following. Additionally, the individual transitions within the five-bursts are phase-shifted in a manner that encodes longitudinal position information (LPOS) into the servo tracks.
The LPOS information is used to keep track of the longitudinal position of data records written onto or read from a tape and is used to locate those data records when the reading or writing process temporarily stops. By detecting the phase-encoded LPOS information, a tape storage system is able to determine the tape position relative to landmarks lengthwise down a tape. The LPOS locations of data files on tape are also stored in the volume control data for use to locate the data files during a later tape cartridge load for reading or for write-appending new files onto the end of the last file written to the tape. The LPOS data is used as the primary positional information for the tape storage servo control system to determine the starting and stopping of a tape and to back-hitch the tape in order to position the read-write heads at the beginning of a data record at the required velocity and track position that allows the start of a new data transfer operation.
LPOS data typically are unable to tolerate any errors. But if a tape drive is reduced to a single servo channel because other servo heads have been smeared or shorted, a single bit error on that channel can cause a Stop Write condition. Thus, the LPOS data has to be able to tolerate some level of errors such that the system can continue to operate with just one good servo head after the occurrence of an error.
One proposed solution to the above-mentioned problem is to append Reed-Solomon parity symbols to an LPOS word. However, the appended parity symbols would increase the length of the LPOS word and would cause problems in synchronizing to the LPOS word because Reed-Solomon words are not base-14 as required by the LTO LPOS format standard in order to allow for synchronization. Another suggested solution would add a base-14 checksum to the LPOS word. But again, this would increase the length of the LPOS word and only provide error detection but not error correction.
Still another solution is proposed in co-pending and commonly-assigned U.S. application Ser. No. 11/205,713 entitled METHOD AND APPARATUS FOR PROVIDING ERROR CORRECTION CAPABILITY TO LONGITUDINAL POSITION DATA, which application is incorporated herein by reference in its entirety. The method and apparatus proposed in the '713 application can detect and correct a one-bit error in an LPOS word.
One proposed technique of providing multi-bit error correction is to use error control coding to compute redundant bits which are then appended to the LPOS word without lengthening the word. However, such a technique reduces the resolution of the LPOS word by reducing the number of bits in the word dedicated to position information.